Incase any body comes across this article, since 2018 platform features have obviously changed and there is now a solution to this.

You can now take advantage of the Anchor (1) and Insert/Edit Link (2) features available in the HTML Editor. 

Anchor: This allow you to declare a specific location within the HTML Field, think if this as a bookmark. 

Inset/Edit Link: Once Anchors have been declared, you can choose to link to an Anchor rather than a URL. 

You could take this a step further by setting up templates with predefined 'Back to the Top' type links. 

Hope this helps!